LCDs are widely used in various modern information products, such as notebooks, personal digital assistants (PDAs), video cameras and the like. The wide usage of the LCD is due to its advantages such as portability, low power consumption, and low radiation. A twisted nematic mode liquid crystal display (TN-LCD) has advantages of low cost and short response time compared with other kinds of LCDs. Therefore, TN-LCDs are particularly popular.
Liquid crystal molecules in a liquid crystal layer of the TN-LCD tilt when an electrical field is applied to the liquid crystal layer. Tilt angles of the liquid crystal molecules are distributed asymmetrically because of the so-called boundary effect. In addition, when light beams pass through the liquid crystal layer, positive phase retardations are generated. This results in viewing angle defects as well as gray-scale inversion in the TN-LCD. To overcome these problems, in general, one or more phase compensation films are added in the TN-LCD.
FIG. 10 is a schematic, exploded side elevation of a conventional LCD. The LCD 100 includes a liquid crystal panel 110, and a backlight module 120 for providing a surface light source to illuminate the liquid crystal panel 110. The liquid crystal panel 110 includes a first polarizer film 111, a first phase compensation film 112, a first substrate 113, a liquid crystal layer 114, a second substrate 115, a second phase compensation film 116, and a second polarizer film 117, disposed in that order from top to bottom. The second polarizer film 117 is disposed adjacent to the backlight module 120. The first phase compensation film 112 and the second phase compensation film 116 are both configured to provide negative phase retardations for light beams passing therethrough, and an optical axis of the first phase compensation film 112 is perpendicular to an optical axis of the second phase compensation film 116.
In operation, light beams provided by the backlight module 120 are transmitted through the second polarizer film 117 and converted to polarized light beams. The polarized light beams pass through the second phase compensation film 116, the second substrate 115, the liquid crystal layer 114, the first substrate 113, and the first phase compensation film 112 sequentially, and then emit from the first polarizer film 111, so as to enable the LCD to function. When the polarized light beams pass through the liquid crystal layer 114, positive phase retardations are generated in the polarized light beams. When the polarized light beams pass through the first phase compensation film 112 and the second phase compensation film 116, negative phase retardations are respectively generated in the polarized light beams. Theses negative phase retardations compensate the positive phase retardations. Thereby, a viewing angle of the LCD 100 is improved.
Refractive indexes of light beams having different wavelengths in the same medium are different, and this impacts optical characteristics of the first and second phase compensation films 112 and 116 as follows. Due to the difference in refractive indexes, while passing through the first and second phase compensation films 112 and 116, negative phase retardations of the polarized light beams having different wavelengths are different, and this further results in differences in polarizing directions of polarized light beams. In particular, polarizing directions of polarized light beams having long wavelengths (for example, more than 700 nm) are not perfectly perpendicular to an optical axis of the first polarizer film 111 when the LCD 100 displays a black image. That is, such light beams cannot be absorbed by the first polarizer film 111 very efficiently, and a light leakage phenomenon is generated. As a result, the phase compensation films 112 and 116 increase the brightness of the black image displayed by the LCD 100, and reduce a contrast ratio of the LCD 100.
It is, therefore, desired to provide an LCD which overcomes the above-described deficiencies.